It is sometimes necessary or desirable to know the partial pressure of certain blood gases and the pH of the blood. One way to accomplish this is to utilize a flow-through housing in an extracorporeal blood loop. In a system of this type, blood flows through the flow-through housing in such a manner as to enable the partial pressures of the blood gases and the pH to be determined.
More specifically, the housing includes two or more dissimilar membranes which allow appropriate components of the blood to pass through the membrane while excluding other components of the blood. The membranes must be dissimilar because each of them must pass and/or exclude different blood components in order that various different blood conditions can be sensed by sensors located on the sides of the membranes not exposed to the blood. For exmple, one of the membrane's may pass oxygen and/or carbon dioxide but will not pass the liquid component of the blood. A second membrane passes an aqueous phase blood component that enables pH to be determined and creates a sterile barrier between the blood and the sensor system.
The blood components that pass through the two membranes are sensed by sensors to provide an indication of pH and the partial pressures of the applicable gases. One technique for accomplishing this is shown in Lubbers et al U.S. Pat. No. 4,003,707.
Although a system of this type performs very well, problems are presented by the mounting of two dissimilar membranes on a membrane support of the housing. If the dissimilar membranes are adhered to the membrane support in end-to-end relationship, the end portions tend to delaminate. This creates turbulence in the blood, tends to rupture the blood cells and to break the sterility barrier. Similar problems exist if the end portions of the dissimilar membranes are overlapped to form a lap joint. In addition, a lap joint, even without delamination, provides an undesirable, rough, nonuniform surface at the membrane interface, and it may be difficult or impossible to find an adhesive which will adhere two membranes of different materials together.
Another problem with the mounting of membranes on a membrane support is in making the membranes taut. This, of course, requires a secure attachment of the membrane to the membrane support so that the membrane can be pulled tightly. Unfortunately, some membranes tend to elongate when wet, and consequently, a membrane of this type may be taut in the initial dry state and relatively loose in a wetted condition. A loose membrane is undesirable because it ripples as liquid flows past the membrane, and this creates erratic results and prevents the membrane from lying flat on the surface of the sensor during operation.